1. Field of the Invention
This invention relates to a method of stabilizing a steel making slag composed mainly of dicalcium silicate, namely, a steel making slag produced in the manufacture of stainless steel, and more particularly to a method of stabilizing a steel making slag wherein the dicalcium silicate in the slag can effectively be stabilized at a state of .beta.-type crystal to prevent the slag from powdering accompanied with crystral transformation in the cooling.
2. Related Art Statement
The slag produced in the refining of stainless steel is composed mainly of CaO-SiO.sub.2 -MgO and particularly contains a large amount of dicalcium silicate having a ratio of CaO to SiO.sub.2 of about 2:1 (abbreviated as 2CaO.SiO.sub.2 hereinafter), so that the powdering phenomenon appears in the cooling and it is impossible to use the cooled slag as ballast. Accordingly, it is an actual state that the slag has been discussed without finding its utility value.
It is known that the above 2CaO.SiO.sub.2 causes a crystal transformation with the change of temperature, which transforms successively from .alpha.-type at high temperature to .alpha.'-type, .beta.-type and .gamma.-type. In the usual case, the crystal transforms from .alpha.-type.fwdarw..alpha.'-type.fwdarw..gamma.-type. In such a crystal transformation, the specific gravity of .alpha.'-type crystal is 3.31 but that of .gamma.-type crystal is 2.97, so that the volume expansion is about 12%, resulting in the powdering to its crystal unit.
However, if specific conditions are given to the cooling, .alpha.'-type crystal may be transformed to .beta.-type crystal as a semistable phase without transforming to .gamma.-type crystal. That is, it is known that if a third component is soluted into the slag, the crystal transforms from .alpha.'-type to .beta.-type without substantially causing the change of the volume and the powdering.
For example, from Handbook of Ceramic Technology, (1973), P1628, published by Gihodo and Refractory Technology, (1962), P388, published by Gihodo, it is clear that B.sub.2 O.sub.3, P.sub.2 O.sub.5, Cr.sub.2 O.sub.3 and the like, whose ionic radius being smaller than that of Si, are effective as a .beta.-type stabilizer. Accordingly, if these stabilizers are added to the slag composed mainly of 2CaO.SiO.sub.2 in the manufacture of stainless steel, it is possible to prevent the slag from powdering.
In Japanese Patent laid open No. 55-128,518 based on such a finding, there is proposed a method wherein the powdering of the slag is prevented by adding 0.4-5% by weight of B.sub.2 O.sub.3 to molten slag produced in the manufacture of stainless steel and cooling the resulting mixture to 300.degree. C. at a cooling rate of not less than 22.degree. C./min.
For example, in the above article of Handbook of Ceramic Technology, there is generally described that the crystal transformation to .beta.-type can sufficiently be caused by merely adding 0.3% by weight of Ca.sub.3 (BO.sub.3).sub.2, coresponding to 0.088% by weight as a conversion value into B.sub.2 O.sub.3, from which it is naturally apparent that the powdering of the slag may be prevented even if the addition amount of B.sub.2 O.sub.3 is slight. In this connection, the above Japanese Patent laid open No. 55-128,518 discloses that boron-containing mineral is excessively added, which means to be due to the fact that the diffusion and the yield of B.sub.2 O.sub.3 are poor. Therefore, such a technic should be improved, considering the fact that the boron-containing mineral is expensive because it is scarcely mined in Japan and is dependent upon the importation from foreign countires.
The size of 2CaO.SiO.sub.2 crystal in the slag composed mainly of the above dicalcium silicate is usually fine within a range of from several microns to about several tens microns. Therefore, when molten slag is stabilized with boron-containing mineral (stabilizer), unless the stabilizer is sufficiently and uniformly diffused into the slag containing crystals of not more than several tens microns, the given effect can not be obtained. However, it is difficult to diffuse the stabilizer into the slag containing the crystal of micron order to even by mechanical agitation because molten slag is a so-called viscous liquid. Consequently, the stabilizer added is partially concentrated and diluted in the slag, resulting in the formation of a powdering portion and a lumping portion containing a .beta.-type and .gamma.-type mixture after the cooling. On the other hand, even if the stabilizer is rendered into a micron size, the addition of powdery stabilizer to the viscous slag is finally caused, so that the same concentrated and diluted portions as mentioned above are produced to make uniform diffusion difficult.